function getXHR(){
            var xhr=null;
            if(window.XMLHttpRequest){
                xhr=new XMLHttpRequest();
            } else if (window.ActiveXObject){
                try{
                    xhr=new ActiveXObject("Msxml2.XMLHTTP");
                } catch(e) {
                    try {
                        xhr=new ActiveXObject("Microsoft.XMLHTTP");
                    } catch (e) {
                        alert("您的浏览器暂不支持Ajax");
                    }
                }
            }
            return xhr;
        }
     
    //ajax与setTimeout排队问题
        function ajax(url,method){
            var xhr=getXHR();
            xhr.onreadystatechange=function(){
                console.log("xhr.readyState:"+this.readyState);
            }
            xhr.onloadstart=function(){
                console.log("onloadStart");
            }
            xhr.onload = function(){
                console.log("onload");
            }
            xhr.open(method,url,true);
            xhr.setRequestHeader("Cache-Control",3600);
            xhr.send();
        }
        var timer=setTimeout(function(){
            console.log("setTimeout");
        },0);
        ajax("http://localhost:63342/project1/js3/html/Ajax%E5%AD%A6%E4%B9%A0.html?_ijt=i9plfqpg77fjialq6b8140t8d3","GET");
console.warn("这里的log并不是最先打印出来的");

为什么会是这么一种结果？哪位大神可以具体解释一下？